1. Field of the Invention
The present invention relates to a magnetic antenna and an antenna device for use in, for example, a radio frequency identification (RFID) system that communicates with external apparatuses via an electromagnetic signal.
2. Description of the Related Art
In RFID systems, which have been increasingly used in recent years, data communication is performed between a mobile electronic apparatus, such as a cellular phone, and a reader/writer each provided with an antenna for information communication. Regarding the antenna provided in a mobile electronic apparatus, in particular, there is strong demand for increased performance and reduced price and size. In response to this demand, a magnetic antenna having a magnetic core disclosed in Japanese Patent No. 3772778 has been developed.
FIG. 1 is a perspective view of the magnetic antenna shown in Japanese Patent No. 3772778. An antenna 10 includes a magnetic core member (magnetic core) 12 and a single electric insulator film (flexible substrate) 14 which has a spiral portion 13a made up of a series of first conductors 13 (coil conductors) formed on one main surface thereof. The other main surface of the electric insulator film 14 has a second conductor 15 formed thereon, and an end of the second conductor 15 and an end of the first conductor 13 are connected to an IC chip 16.
However, since the structure shown in Japanese Patent No. 3772778 is a structure in which the flexible substrate is folded at the center so as to sandwich a magnetic core, basically, the antenna only is only coupled by magnetic flux that comes from a direction substantially parallel with the main surfaces of the magnetic core. Hence, magnetic flux coming from a direction substantially perpendicular to the main surfaces of the magnetic core is not able to pass through the loop plane of the coil conductor from one side to the other side, thereby causing very weak coupling.
Further, the opening of the antenna becomes smaller since portions of the coil conductor occupy part of the surface of the magnetic core, through which magnetic flux passes. This also causes the coupling to become weaker.